An image sensor provides a grid of pixels, such as photosensitive diodes or photodiodes, reset transistors, source follower transistors, pinned layer photodiodes, and/or transfer transistors for recording an intensity or brightness of light. The pixel responds to the light by accumulating a charge—the more light, the higher the charge. The charge can then be used by another circuit so that a color and brightness can be used for a suitable application, such as a digital camera. Common types of pixel grids include a charge-coupled device (CCD) or complimentary metal oxide semiconductor (CMOS) image sensor (CIS). Backside illuminated (BSI) image sensors are used for sensing a volume of exposed light projected towards the backside surface of a substrate. The pixels are located on a front side of the substrate, and the substrate is thin enough so that light projected towards the backside of the substrate can reach the pixels. Backside illuminated sensors provide a high fill factor and reduced destructive interference, as compared to front-side illuminated sensors.
As part of the BSI manufacturing process, CMOS wafers are bonded to silicon or glass carrier substrates. Once this bonding process is completed, the wafer is thinned down to several microns using techniques such as grinding, polishing and/or etching. Problems with the backside illuminated image sensors are bonding quality and wafer thinned-down process, which have been the critical bottleneck for mass production. Especially, the thinned-down process followed by backside implantation and wet etching processes generate other issues related to silicon surface protection.
Accordingly, there is a need for an improved method of fabricating such to avoid the shortcomings of the conventional process.